The impact of soil carbon to nitrogen ratios on disease suppression

A foundational goal of regenerative agriculture management practices is to increase the volume of carbon that is cycled through soil systems. Not just statically stored in soils, but cycled through. The more volume of carbon that is cycled, the more robust the soil microbial community becomes, the more efficient plant photosynthesis becomes, and the better the entire ecosystem functions.

When more carbon is cycled in different forms, microbial balance and activity shifts to match, which results in changing the quantity of nitrogen that is sequestered, and the quantity of phosphorus, sulfur, silicon, and trace minerals that are released from the soil mineral matrix.

When abundant carbon is cycled, soil biology has the food sources required to fix all the nitrogen they require from the atmosphere, and no additional N needs to be added. This also results in a change of the dominant direction of N mineralization to be primarily nitrate or ammonium, which influences disease suppression and crop nitrogen sufficiency.

Here are some important thoughts on this topic Don Huber shared:

John: What is the impact of carbon-to-nitrogen ratios on both disease-suppressive soils and also on yield?

Don: The carbon-to-nitrogen ratio depends on the carbon source.

That got me in trouble with my first publication in plant pathology. I challenged the carbon-nitrogen ratio hypothesis. People were saying, “If you have a 12:1 versus a 40:1 ratio, you’ll always have a disease relationship.” And I demonstrated that it’s not the carbon-to-nitrogen ratio. It’s the form of nitrogen that is involved in that ratio.

You can take different crop residues or a different cropping sequence, and it’s the effect of that sequence on the form of nitrogen that determines what the disease reaction is. And, of course, the effect of that form of nitrogen quite often is an effect on manganese or zinc or copper or other nutrients, along with the form of nitrogen.

Carbon-nitrogen ratios work if you’re working with the same nutrient source or crop residue and then varying the nitrogen ratios by either harvesting plants when they’re greener or harvesting plants drier—when you have wider carbon-nitrogen ratios. But the carbon-nitrogen ratio per se isn’t the factor that’s involved there. It’s the effect of that ratio on the form of nitrogen and the other minerals that are involved—such as manganese or zinc or iron or copper—that are critical for particular physiological processes.

P.S. I had an interesting discussion with Koen van Seijen on the Investing in Regenerative Agriculture podcast that just released. Much of our discussion revolved around the question, “How would I invest a billion dollars in accelerating the adoption of regenerative ag?” You can find it here.

The effectiveness of microbial inoculants in fixing nitrogen

Soil biology can ‘fix’ and supply more nitrogen, and faster, than they are often given credit for.

The wheat field section on the right received an October application of AEA’s soil primer, which includes bacterial inoculants and stimulants. By March the following year, soil analysis reported an additional 80 units of N available for the crop.

To achieve these results, the soil must have adequate microbially active carbon, good gas exchange, and good moisture levels.

Soil microbial populations can regenerate quickly when given the right environment and support. Regenerating soil health can be accelerated to a few years, it is not a process that needs to take decades to achieve a significant turn around.

2020-05-28T07:05:35-05:00May 28th, 2020|Tags: , , , |

How the form of nitrogen influences insect feeding

The form of nitrogen influences not only the pathogenicity of soil borne fungal diseases, but also susceptibility to insects.

From the podcast interview with Larry Phelan.

John: As you were looking at these plant-insect dynamics in the field and developing your hypothesis of biological buffering, what was something that surprised you?

Larry: The one thing that was particularly exciting to us was when we took the next step and tested this idea of mineral balance resulting from this biological buffering of organic matter. We started growing plants hydroponically so that we could vary the proportions of different nutrients. The hypothesis we were testing was actually that when the plant was in good mineral balance, you would get both good growth and resistance to insect attack. And then as the plant moved out of balance nutritionally, you would see plant growth go down and insect performance actually go up. And then, ultimately, as the plant was way out of balance nutritionally, we expected the plant to not grow very well and the insects not to do very well either because of the poor host plant.

We tested this with a number of different combinations of nutrients, and the one that was most dramatic—that actually supported this prediction—was looking at soybeans in which we varied the ratios of ammonia to nitrate in the plant. We provided it all the nutrients that it needed in constant levels for all the plants. What we were testing was different ratios between these two different forms of nitrogen. And what we found was that as we increased the amount of ammonia up to about 30 percent, we saw the best plant growth. That ratio of 30 percent ammonia and 70 percent nitrate is where we got the best plant growth.

And then, when we looked at insects, we plucked the leaves off of these plants and then fed them to insects to see how the insects grew. The particular insect we were working with was the Mexican bean beetle. When we fed these leaves to the Mexican bean beetle, we saw just the opposite response. In other words, where the plant was out of bounds nutritionally and not growing very well, that’s where the insects grew the largest and that’s where we saw the best survivorship. But as we moved towards that 30 percent ammonia level, where the plants were growing their biggest, insect survivorship dropped from about 90 percent down to about 30 or 40 percent.

It was a very dramatic effect—even more dramatic than what we were expecting. When we followed up on this and measured the levels of free amino acids in these plants, it was consistent with the prediction. In other words, those plants that were not growing as well, that were out of balance in terms of this ratio, had much higher levels of free amino acids relative to that 30 percent ammonia plant.

2020-05-05T05:16:18-05:00May 12th, 2020|Tags: , , , , , |

How the form of nitrogen influences disease suppression

Nitrate and ammonium nitrogen have dramatically different impacts on soil biology and possible pathogens. Some pathogens are enhanced by nitrate and suppressed by ammonium. Others are the exact opposite. Most (but not all) soil-borne pathogens are enhanced by the presence of nitrate. This corresponds to the impact of reduced vs oxidized environments, since ammonium is the reduced form of nitrogen, and nitrate is the oxidized form of nitrogen. In general, reduced environments are very disease suppressive, and oxidized environments are disease enhancing.

I have had many discussion about this topic with Don Huber, inlcuding this one on a podcast interview.

John: What are the impacts of nitrogen and nitrogen applications on developing disease-suppressive soils?

Don: Most of your soil organisms are hungry for two things. One of them is nitrogen. The other is carbon. When you change either of those nutrients, you see tremendous stimulation of a lot of organisms in the soil, depending on what your source of nitrogen is.

One of their primary pathogens on a lot of vegetable crops in Florida is Fusarium oxysporum. It’s a vascular Fusarium. Growers can get pretty much complete control by using nitrate nitrogen and calcium. If they can stimulate nitrification, or if they apply nitrate nitrogen, potassium nitrate, or calcium nitrate—and also use some liming (lime = an oxidizer) to get their pH up—they have fairly effective control of Fusarium wilt diseases.

With tobacco, where we use fumigation to control some of the soil-borne diseases, you should have at least 30 percent of your nitrogen as nitrate nitrogen. If you don’t—if the plant is taking up all ammonium nitrogen—you can get into a carbon deficit because the plant detoxifies the nitrate or the ammonium nitrogen by combining with photosynthate from photosynthesis; that provides the carbon base for those amino acids. Then the nitrogen is translocated as the amino acid. If you don’t have enough nitrate nitrogen present to buffer against an ammonium source—if you’re going to get fumigation, because our soil fumigants tend to knock out nitrifying organisms—or if you don’t get nitrification there, it stabilizes in the ammonium form. It can be a drain on the carbon and the energy availability until that ammonium is detoxified and utilized by the plant.

John: I’m thinking about your description of how many soil-borne pathogens—soil-borne fungi such as Fusarium and Verticillium, for example—are dependent on oxidizing manganese and limiting manganese absorption by the plant. If that were the case, what would be the impact of adding ammonium to such an ecosystem—where you have a reduced form of nitrogen? What would the impact of the ammonium be on these soil-borne pathogens?

Don: On that group of pathogens, you will see a tremendous reduction in disease—with the ammonium nutrition. I wrote a chapter in one of the annual reviews on the impact of the form of nitrogen. You’ll see a tremendous benefit in different organisms by the form of nitrogen that the plant is predominantly supplied with. If you modify the environment so that those soil microorganisms make those conversions, you’ll see that the form of nitrogen will be available for the plant.

So, it’s important that you have both the management tools as well as the form. Most of our soil’s nitrification takes place very rapidly, so you need to do something to inhibit nitrification. We can do it biologically—we can modify the speed of that reaction so that we can increase the amount of nitrogen in nitrate or ammonium. When it’s taking ammonium nitrogen, you have a reducing environment. That reducing environment is favoring manganese-reducing organisms so that there will almost always be an increase in manganese availability for the plant—when you predominantly use an ammonium form of nitrogen. The more rapid, oxidative form is the nitrate source of nitrogen.

Nutritional differences in insect susceptible plants

Healthy plants are completely resistant to insects.

We have observed this to be true in the field on many occasions. Over time, more connections come to light describing the scientific reasons for how this occurs.

Larry describes their pioneering research seeking to identify the nutritional differences between insect resistant and insect susceptible crops in our podcast episode here.

So, if you have an imbalance—if you have too much nitrogen relative to potassium—what happens is you get the buildup of free amino acids in those plants. And insects love free amino acids. They’re a very digestible source of nutrients for them because they’re also highly limited by nitrogen.


John: With some of the original research that you did—comparing the insect pressure on the organic farms versus the nonorganic farms—this is actually something that I often hear from organic growers, but I haven’t uncovered a lot of research where people have actually tried to do a comparison and evaluate the differences. What were the differences that you were seeing? What really stood out for you?

Larry: Again, keep in mind that this was twenty to twenty-five years ago. There’s been a lot more research in organic systems now. But at the time, organic farming was considered very unscientific, and more New Age, and not amenable to large-scale production. And so I was working with a lot of organic farmers who were basically doing their own research—not necessarily replicated research as we would do at the university, but trying different things and seeing what worked on their farm. This is the context in which we got started.

I actually received a lot of criticism at the time—that what I was doing was not very productive, or not the direction I should be going as a non-tenured faculty member. So, we went to these organic corn and soybean farmers, and the ones we worked with had animals integrated into their system as well. First of all, we just did a census of their corn, looking at the levels of European corn borer damage in their fields versus their conventional neighbors. And, as I’m sure you and many of your listeners have often heard, I kept hearing this idea that if we have healthy soil, then we’re going to have a healthy plant and that insects don’t like healthy plants—that’s why they don’t see the damage.

Although that in itself is not a very scientific statement, we could reformulate it as a hypothesis that we could test empirically. And so we collected soils from these organic farms and then went right across the street to a conventional neighbor, collected their soils, and brought these soils into the greenhouse and planted them to corn. And we fertilized each of them either with an organic fertilizer, like manure or compost, or with a chemical fertilizer.

What we were interested in figuring out was, first of all, whether the insects could tell a difference in these plants. And if they could tell a difference, was it associated with that short-term effect of the type of fertilizer we used or that long-term effect of that history of management and how that impacted the soil community. And so, after setting these plants up and letting them grow to a certain stage, we released European corn borer females that had been mated into the greenhouse. All of this was replicated and randomized.

And we just let them loose to see where they would lay their eggs. And what we found very consistently—we actually repeated this experiment, I think, with four or five different pairs of soils from farms—was that if the plants were growing in a soil from an organic farm, irrespective of the fertilizer we applied, they received relatively few eggs. Whereas if the plants were growing in a soil from a conventional farm, sometimes they would receive a lot of eggs and sometimes they would receive only a few eggs.

This gave rise to this concept that I call biological buffering. The way we envision this—and this was our working hypothesis—was that in those organic systems, where you have a recurring influx of organic matter into those soils, either in terms of cover crops or plant manures or animal manures, you create this soil community that is beneficial to the plant. And when nutrients then go into that system, they get absorbed by this microbial community and then they release those nutrients very slowly over time. As a result, we hypothesize that those plants are in better mineral balance than when you’re putting down high levels of nutrients.

Why this would be important is because plants are almost always limited by nitrogen levels. They don’t have mechanisms for dampening the levels of nitrogen that they take up. They’re going to take up whatever they can get. And in this context of putting down inorganic, highly soluble nitrogen sources, often these plants are taking up much higher levels of nitrogen than they are really set up to deal with.

We then hypothesized that in this situation, those plants would tend to accumulate the simple compounds. When you have an imbalance of nutrients—let’s say nitrogen and potassium—if you think about those two elements, nitrogen is of course very important in terms of protein synthesis and potassium is important in terms of converting amino acids into protein. So, if you have an imbalance—if you have too much nitrogen relative to potassium—what happens is you get the buildup of free amino acids in those plants. And insects love free amino acids. They’re a very digestible source of nutrients for them because they’re also highly limited by nitrogen.

In situations where you have these imbalances, that plant becomes very nutritious for the insect; whereas in the plant that is in better mineral balance—one that’s getting its nutrients relatively slowly—that metabolic machinery of the plant is able to act more efficiently. As amino acids and sugars are produced in the plant, they are more immediately converted to the less digestible and more complex building blocks of the plant, like proteins and starches and cellulose and that sort of thing.

Increasing Nitrogen use efficiency

Not all forms of nitrogen are created equal. A pound of nitrogen in one form will produce a completely different crop response than a pound of nitrogen in a different form. This is why organic growers often describe requiring only a fraction of the N requirement to produce a bushel of a given crop when compared with mainstream N applications.

The ultimate ideal is for plants to absorb amino acids and proteins directly from the soil microbial population and in the form of microbial metabolites. These forms of nitrogen contribute a lot of energy to plants, much more than. That represented by the N they contain. 

The second most efficient form of N for most crops to absorb is urea, or amine nitrogen. 

The third most efficient form of N for crops to absorb is ammonium.

The least efficient form of N for crops to absorb is nitrate. Plants must use a significant amount of their photosynthetic energy to convert nitrate to amino acids and proteins. When a corn crop absorbs 80% of it’s N requirement, it requires 16% of it’s total photosynthetic energy just for nitrate conversion (Marschner) A plant also requires three times more water to convert nitrate to amino acids as compared to ammonium. These are just the beginning items on a long list of reasons why you want plants to absorb only minimal amounts of nitrate, and obtain the majority of their nitrogen from other forms, preferably directly from the microbial population.

Ultimately the goal is to develop soil microbial populations that can deliver 100% of a crops nitrogen requirement every year. This is a very realistic and achievable goal. Only if you stop killing them with synthetic N applications in the first place, of course.

While on the pathway to reducing N applications, the first step is to make certain that any applied N is rapidly consumed by the soil bacterial population, and converted to microbial proteins and amino acids. These microbial forms of N are not leachable and are available to plants even when there is less water in the soil profile.

To convert applied N, either liquid 32-0-0, liquid 28-0-0, or liquid urea 21-0-0 (the most efficient of the liquid N sources) we simply need to provide the food sources and stimulants for biology to rapidly consume the applied nitrogen. 

Here is a recipe we use on a lot of acres, very effectively:

1. 3% of the total solution (either weight/weight or volume/volume) should be humic acid. We use HumaCarb.

2. Add ATS, ammonium thiosulfate 12-0-0-26S to produce a 10:1 nitrogen to sulfur ratio in the final solution.

3. Add a carbohydrate source, we use Rejuvenate at 3% of the total solution

4. Add molybdenum, needed for the nitrate reductase enzyme. We use Rebound Molybdenum at a pint or a quart per acre.

It isn’t realistic to make universal recommendations, given the wide variability in soils, crops, and management practices, but we commonly observe that growers reduce nitrogen application rates by 30%-40% or more in the first year and produce the same or higher yields as compared with controls when using this combination. Use sap analysis to diagnose precisely whether the crop has adequate N, we don’t live in a world where we have to guess and be uneasy. Many times, we use this approach, and growers are amazed that their crops constantly show they have abundant, even surplus nitrogen. This is a start down the path to producing all your own N in the soil profile. We walk around in 78% N, the only reason we buy any is because we have destroyed the capacity of our soils to produce it’s own. 

The same solution can be used for dry N applications if you can get it applied to the dry product. 

Nitrogen management is a big topic, look for more thoughts on this in the future. 

2020-03-16T14:02:07-05:00February 8th, 2020|Tags: , , , |

Fixing nitrogen without Legumes

Legumes don’t fix nitrogen.

Only bacteria do.

And there are many more bacteria capable of fixing nitrogen than those associated with legumes.

We just need to stop killing them, begin encouraging and feeding them, and our soils and crops can be supplied with 100% of the nitrogen requirements at the highest yield levels.

A new report describes some of the rhizobia and other organisms found in the rhizosphere of plants other than legumes1.

The important part, of course, is that these biology need an abundant energy source to be able to fix nitrogen. The more energy they have available, the more N will be fixed. A large part of their energy during the growing period is supplied by plant root exudates. When we have plants with optimum photosynthesis, producing large volumes of exudates, much more N can be sequestered, which leads to higher yields.

1. Yoneyama, T., Terakado-Tonooka, J., Bao, Z. & Minamisawa, K. Molecular Analyses of the Distribution and Function of Diazotrophic Rhizobia and Methanotrophs in the Tissues and Rhizosphere of Non-Leguminous Plants. Plants 8, (2019)

2020-03-16T13:34:25-05:00December 9th, 2019|Tags: , |
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